LCDs are used as display screens in many common applications including digital clocks, microwaves, laptop computers, calculators and other electronic devices LCDs offer advantages over luminescent display technologies such as cathode ray tubes (CRTs) because they are lighter, thinner and require less voltage and power to operate.
LCD screens have good picture quality and contrast when viewed directly, i.e. at an angle that is perpendicular, or normal, to the plane of the screen. However, picture quality and contrast decline as the viewing angle increases. Image degradation occurs because the LC cell is birefringent and splits the entering light beam into two light beams (ordinary and extraordinary) that propagate through the LC cell in different directions and different speeds. The ordinary and extraordinary rays move in different planes, at different speeds and have different indices of refraction (no and ne, respectively). The ordinary ray travels in a direction parallel to the direction of the liquid crystals, while the extraordinary ray travels in a direction perpendular to the direction of the liquid crystals. Since light beams at different angles experience different retardations, poor image quality occurs at higher viewing angles. Optical compensation films are used to improve the image quality at high viewing angles by correcting for the variation between ordinary and extraordinary indices of refraction of the light beams passing through the LC cell.
An IPS-LCD device is a type of LCD with LC molecules that lie in-plane, i.e. parallel to the substrate and parallel to each other. An IPS-LCD generally includes a liquid crystal (LC) layer having positive or negative dielectric anisotropy a pair of glass substrates sandwiching the LC layer, and a pair of polarizing films sandwiching the glass substrates and the LC layer together. The LC layer is applied with a lateral electric field that is parallel to the substrates to control the direction of the LC molecules for image display. In the off position, the molecules of liquid crystal lie parallel to the glass substrates and to each other and the cell's electrode pair. When a voltage is applied across the electrode pair, the LC molecules can all rotate freely through 45° to align themselves with the field, while remaining parallel to the substrates and other molecules above and below them.
Since IPS-LCDs have molecules that are aligned in-plane, light moving through the LC parallel to the plane of molecules plane of the has a higher index of refraction than light moving in the direction perpendicular to the molecules. Thus, light passing through the IPS LC cell has the relationship n∥>>n⊥, or ne>>no. In other words, light moving parallel to the LC molecules in the x or y directions on a Cartesian plane has a higher index of refraction than light moving perpendicular to the LC molecules in the z direction.
Optical compensation films with positive birefringence (positive C-plates) are used to compensate for the imbalance created by an IPS-LCD by providing an optical compensation film wherein n⊥>>n∥, or no>>ne. The optically anisotropic units in positive C-plates are aligned perpendicularly to the substrate in general so that light moving in the z direction has a higher index of refraction than light moving in the x or y direction.
Birefringence, Δn, measures the difference between the indices of refraction of the ordinary and extraordinary rays:Δn=no−ne=n⊥−n∥An IPS-LC's cell has a birefringence characterized by n∥>n⊥. Thus, the positive C plate needs a birefringence characterized by n⊥>>n∥ to compensate the IPS-LC cell. The greater n⊥ is compared to nil in a positive C plate, the greater the difference in their refractive indices and the higher the birefringence of the compensation film. High birefringence in a compensation film creates a more effective C plate.
The concept of a positive C-plate is known in the art as are some compositions of positive C-plates such as polystyrene. However, polystyrene compensation films are limited because n⊥ is only slightly higher than n∥. Thus, polystyrene films are poor compensators for LC cells, and a relatively thick polystyrene film is needed to appreciate any significant LC cell compensation.
Positive C-plates made of poly(vinylcarbazole) are also known in the art. Poly(vinylcarbazole) compensation films have higher birefringence than polystyrene films, but are photolytically unstable and decompose when exposed to light. Thus, poly(vinylcarbazole) compensation films are not a commercially viable positive C-plate because they are unstable.
Current commercial positive C-plate materials require expensive post-synthesis processing such as locking liquid crystal molecules into a perpendicular alignment by photopolymerization. Without the photopolymerization step, the film would not have the proper birefringence to function as a compensation film.
US Patent Application No. 2005/0200792 A1 discloses an in-plane switching liquid crystal display comprising a negative biaxial retardation film and a positive C-plate as a viewing angle compensation film.
US Patent Application No. 2005/0140900 A1 discloses an IPS-LCD comprising a positive A-plate and a positive C-plate. No chemical composition of the positive C-plate is disclosed.
US Patent Application No. 2005/0270458 A1 discloses a multilayer optical compensation film comprising optically anisotropic first and second layers, wherein the second layer includes amorphous polymer with a glass transition temperature above 160° C., and the indices of refraction satisfy the relations |nx−ny|<0.001 and nz−(nx+ny)/2>0.005.
U.S. Pat. No. 5,189,538 discloses a liquid crystal display comprising a film having light transmission properties that includes a uniaxially stretched polymer film with positive intrinsic birefringence.
U.S. Pat. No. 6,115,095 discloses an in-plane switching LCD comprising a first compensation layer having positive uniaxial, optical anisotropy, and an optical axis extending perpendicularly to the substrate. No chemical compositions of the compensation layer are disclosed.
U.S. Pat. No. 6,175,400 discloses a broadband cholesteric optical device having a broadband cholesteric layer and a compensator in the form of a positive birefringence film whose optic axis is substantially perpendicular to the film.
US Patent Application No. 2005/0037155 discloses a retardation plate obtained by laminating a homeotropic liquid-crystal film and a photopolymerizable liquid crystalline compound to a stretched film having a retardation function.
US Patent Application No. 2005/0122456 discloses an optical film with a substrate without a vertical alignment layer and a homeotropic alignment liquid crystal film layer formed on the substrate. The homeotropic alignment liquid crystal film layer comprises a homeotropically aligned liquid crystal polymer
Thus, there remains a need for an optical compensation film having a large positive birefringence value without being subject to expensive processes such as stretching, photo irradiation, and heat treatment. The optical compensation film should be stable at ambient conditions, optically transparent, have low color, and be easy to apply onto a substrate.